Thermoelectric materials interconvert thermal gradients and electric fields for power generation or for refrigeration. Thermoelectrics currently find only niche applications because of their limited efficiency, which is measured by the dimensionless parameter ZT-a function of the Seebeck coefficient or thermoelectric power, and of the electrical and thermal conductivities. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another. Several groups have achieved significant improvements in ZT through multi-component nanostructured thermoelectrics, such as Bi(2)Te(3)/Sb(2)Te(3) thin-film superlattices, or embedded PbSeTe quantum dot superlattices. Here we report efficient thermoelectric performance from the single-component system of silicon nanowires for cross-sectional areas of 10 nm x 20 nm and 20 nm x 20 nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including ZT approximately 1 at 200 K. Independent measurements of the Seebeck coefficient, the electrical conductivity and the thermal conductivity, combined with theory, indicate that the improved efficiency originates from phonon effects. These results are expected to apply to other classes of semiconductor nanomaterials.

Trends in the singlet-triplet state-splittings of substituted carbenes are found to be reproduced accurately with a very simple level of ab initio theory. The minimum balanced description, GVB( 1 /2), with a simple basis set yields singlet-triplet gaps that correlate linearly with available accurate values. This linear relationship is exploited to predict the state-splitting for the remaining members of the title series of carbenes. The magnitudes of the singlet-triplet splittings can be rationalized in terms of the charge on the carbenic carbon atom as well as r-donation from the substituents. The correlation with charge, in conjunction with electronegativity equalization, permits singlet-triplet gaps to be predicted for arbitrary simple carbenes using only a hand calculator. Since both charge and r-donation are important in determining the energy gaps, we conclude that a-donation and r-backbonding act synergistically.

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A parametrized reactive force field model for aluminum ReaxFF Al has been developed based on density functional theory ͑DFT͒ data. A comparison has been made between DFT and ReaxFF Al outputs to ascertain whether ReaxFF Al is properly parametrized and to check if the output of the latter has correlation with DFT results. Further checks include comparing the equations of state of condensed phases of Al as calculated from DFT and ReaxFF Al . There is a good match between the two results, again showing that ReaxFF Al is correctly parametrized as per the DFT input. Simulated annealing has been performed on aluminum clusters Al n using ReaxFF Al to find the stable isomers of the clusters. A plot of stability function versus cluster size shows the existence of highly stable clusters ͑magic clusters͒. Quantum mechanically these magic clusters arise due to the complete filling of the orbital shells. However, since force fields do not care about electrons but work on the assumption of validity of Born-Oppenheimer approximation, the magic clusters are therefore correlated with high structural symmetry. There is a rapid decline in surface energy contribution due to the triangulated nature of the surface atoms leading to higher coordination number. The bulk binding energy is computed to be 76.8 kcal/mol. This gives confidence in the suitability of ReaxFF for studying and understanding the underlying dynamics in aluminum clusters. In the quantification of the growth of cluster it is seen that as the size of the clusters increase there is preference for the coexistence of fcc/hcp orders at the expense of simple icosahedral ordering, although there is some contribution from distorted icosahedral ordering. It is found that even for aluminum clusters with 512 atoms distorted icosahedral ordering exists. For clusters with N Ն 256 atoms fcc ordering dominates, which implies that at this point we are already on the threshold of bulklike bonding.

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